The present invention relates to a signal transmission circuit used in a semiconductor integrated circuit.
In integrated circuits, it is often required to deliver a signal over substantially the total area of the semiconductor chip, or to transmit a signal from a first point on the chip to a second point far distanced from the first point. This being the case, a relatively long transmission line is used. This long transmission line is usually composed of a resistive metal having the smallest possible resistance, such as aluminum. A large stray capacitance typically exists between the metal transmission line and the silicon substrate or other such conductive layer. The longer the transmission line, the larger the stray capacitance. This stray capacitance delays propagation of a signal through the transmission line, and hence adversely influences the operating characteristics of the integrated circuit.
As shown in FIG. 1, an input signal is supplied to an input pad 1 formed on an IC chip 2. The signal is further transmitted to an internal circuit 5 through a protective resistor 3 and a long interconnection line or a signal transmission line 4. The protective resistor 3 protects the internal circuit 5 from an abnormally high voltage accidentally applied to the input pad 1. For this reason, the protective resistor 3 has a high resistance, usually 1 to 2 kilo ohms. The resistor 3, in combination with the stray capacitance associated with the transmission line 4, operates to greatly delay propagation of the signal through the transmission line 4.
Referring to FIG. 2, the stray capacitance associated with the transmission line 4 is the sum of the capacitance C1 between the transmission line 4 and the semiconductor substrate 6, and the capacitance C2 between the transmission line 4 and its adjacent interconnection lines 7. In an integrated circuit of a high circuit density, the distance between the lines 4 and 7 is short. The stray capacitance associated with the transmission line 4, therefore, is effectively influenced by the stray capacitance C2. Thus, as the interconnection lines are laid closer to each other, the stray capacitance associated with the transmission line 4 becomes larger, lowering the signal propagating speed.